Composite metal article



United States Patent 3,064,337 COMPGSITE METAL ARTHILE Milton B. Hammond and Glade B. Bowman, Edgeworth,

Pa., assignors to Rockwell-Standard Corporation, Coraopolis, Pa, a corporatien of Pennsylvania No Drawing. Filed May 19, I958, Ser. No. 735,950 11 Claims. (Cl. 29-1965) This invention relates to electroplating and more particularly to a novel process and the plated product produced thereby. The present application forms a continuation-in-part of copending Hammond et al. application, Serial No. 618,679, filing date October 29, 1956, now Patent No. 2,989,446, and assigned to the assignee of the present application.

In plating ferrous articles and other basic metal members with electro-deposits of nickel-zinc alloy, it is desirable to apply a predetermined thickness of alloy of proportions which we can term zinc-rich as used now and hereinafter to mean more than 50% zinc and less than 50% nickel. Preferably the nickel content is about 13% to about 24% nickel and the balance is zinc. This alloy deposit is highly suited as a plating on both ferrous and non-ferrous articles, the latter including brasses and die castings particularly. In connection with the former, the zinc-rich alloy deposit has remarkable properties in increasing the corrosion resistance of the ferrous base article, and the advantages of using this deposit and some explanations advanced therefor are more completely disclosed in Schantz Patent No. 2,419,231. The Schantz patent further discloses an electroplating bath of specified compositions which have proved very satisfactory in producing the zinc-rich deposit on the article.

The bath employed in the present invention is very similar to the just mentioned plating bath according to the Schantz teaching which employs unidirectional direct current only therein to continuously cathodically plate a ferrous article with an alloy deposit which is zincrich, containing approximately 85% zinc and 15% nickel. The zinc-rich deposit offers good corrosion resistance on iron or steel as already mentioned, but has the deficiency of being practically impossible commercially of taking a chromium plate or a nickel plate thereover which will have good adherence to the zinc-rich deposit. The difliculty with nickel-zinc alloy which is zinc-rich is that it dissolves in electrolyte baths generally used in commercial chrome plating or nickel plating, and continues to do so in spots at least, even when the plating current is flowing.

In the present invention we retain the advantages of corrosion proofing generally according to Schantz Patent 2,419 ,231, but in doing so we produce a composite form of deposit to which a chrome plate or nickel plating 'readily adheres. In accordance with our invention, in

a first step we generally follow the teachings of unidirectional plating current in the alloy plating bath of the Schantz patent to produce a zinc-rich deposit on the article and thereafter in a subsequent step two we employ periodic reversal of current to apply upon the zinc-rich alloy a nickel-rich alloy deposit of a particular character to which nickel or chromium plate will satisfactorily adhere. The feasibility of producing this nickel-rich alloy comes from our present discovery that in one bath, merely by controlling the current density, the current duration, and the current direction, we can produce the unexpected and unusual result that nickel-rich alloy commences to over coat the zinc-rich alloy which had previously come from the same bath. In distinction to the term zinc-rich which is already defined, we intend the ter-m nickel-rich to mean more than nickel and less b I 1C6 than 50% zinc. In distinction to ordinary cathodic unidirectional plating (DO), We intend the term periodic reversa (P.R.) to mean the utilization of the direct current in one direction for a short period in which the article is anodic, followed by a short period in which the current is switched in reverse to make the article cathodic, followed by another short period in which the current is again reversed to make the article anodic, and continuing to switch the direction of current flow until completion of the period desired. a

It is not clear why the proportions of the alloy deposit being plated onto the article become changed incident to RR. switching so that the zinc-rich layer is over coated with the nickel-rich layer and vice versa, nor is it clear why the resulting nickel-rich layer will receive and oifer particularly good adherence to a subsequent chrome plate or nickel plate. However, some tests have indicated a provisional or working hypothesis. When an article to be plated is immersed in an electrolytic bath which under normal current density such as 50 amps. per sq. ft. will plate out a zinc-rich alloy deposit thereon, and when the current density employed is below approximately 20 amps. per sq. ft. for some articles, the alloy deposited thereon results in being nickel-rich varying from 50% to nickel, depending on other variables, with the balance being zinc. Further, when that article is cathodically plated with the alloy for a period which is interrupted with a reversal of current to give the article a short anodic treatment resulting in its polarization, and when the cathodic treatment is immediately or subsequently resumed so as to depolarize and restart plating of the article at normal current density, the efiective proportion of the current density immediately available for actual plating is considerably lower for the initial part of the period in which all or a large part of the electrical energy is being consumed in the immediate problem of depolarization, that is, dispelling gas bubbles from the passive surface of the article in order to reactivate it and to resume normal cathodic plating. Thus, the effective part of the normal current density during the initial cathodic phases of RR. plating may be so low that only nickel-rich deposits initially occur and these normal total current densities of 50 amps. per sq. ft. referred to are conventional in bumper plating work, for instance, in which automobile bumpers with an average area of 10 sq. ft. are subjected to 500 amps. current input on the meter.

Longstanding suspicions have existed in the art with I regard to polarization effects, and criticisms of their harmful qualities in electroplating work, particularly reverse current operations, have been suggested heretofore as in Holt Patent 1,534,709. The disadvantages of polarization are explicitly set forth in the Holt patent which teaches periodic current reversal to combat and eliminate those disadvantages, whereas instantly it would appear that the polarization eifects are actually being harnessed to produce the result presently intended. In any case, the theory of reversal of proportions as a result of unidirectional current followed or preceded by polarization effects of RR. current treatment, and the further theory of an alloy deposit of metals which is produced by applications of RR. current in a manner so as to take chrome plate or nickel plate directly thereover where not previously commercially possible, are used in this specification only as convenient bases for explaining the invention.

The most unexpected and improved results have been obtained by employing one or more intervals of RR. current cycles of this nature in electroplating nickel-zinc alloy. In the present inventionand, generally in accord- 4 3 ance with the teachings of the mentioned Schantz Patent 2,419,231, we employ a bath containing:

gm.ll. z./gal.

NiCl fiHqO 240-300 3240 Zn 2. 7 I 90420 1216 The bath also contains 13% by volume of glacial acetic acid or other suitable buffer, for instance, the buffer may be salts of acetic acid and also formic, citric, boric, or tartaric acid or the salts thereof. The bath further contains by volume 1% of a wetting agent of composition not specifically known, but commercially available as the Udylite (Detroit, Michigan) No. 22 Non Pitter. The contents of the NiCl -6H O and the ZnCl are equivalent respectively to 8-10 ozs. per gal. of nickel metal and 6 -8 ozs. per gal. of zinc metal. The latter zinc metal may be present in the bath in the form of zinc sulfate instead of or with the zinc chloride but at least 50% of the total of the nickel and zinc salts must be chloride. The pH of the bath is 1v to 3 and preferably 2.0-2.5, the 'pH being adjusted to within this range by the addition of hydrochloric acid to lower the pH and by theaddition 'of'either nickel or zinc carbonate or zinc oxide to raise it. The bath temperature range is 70 F .-'1 65 F., and preferably the bath is between about 135 f F. and 145 F. The cathode current density is20-600 amps. per sq. ft., but preferably the cathode current. density is 50-60 amps. "per sq. ft. The article itselfc'onstitutes the cathode (literally anode during the anodic phases of the current reversal cycle) and the anodes themselves may take several satisfactory forms, but in any case the electrolyte is kept in circulation in well-known manner while the articles are being treated, except when plating upon wire or strip moving through the electrolyte such circulation may not be necessary.

Preferably, nickel anodes are used or else separate nickel and zine anodes are used having their surface areas relatively proportioned on the same ratio as the nickel and zinc in the desired zinc-rich alloy. In the latter case it is desirable to employ a plurality of the zinc anodes and a plurality of the nickel anodes and to position themproperly on the anode support in a spaced relationship to get'the current distribution desired. Instead of using the separate zinc and nickel anodes, cast or rolled alloy anodes may be used having the metals inabout'the same proportions as inthe zinc-rich alloy being plated out or, if desired, having the metals in substantially equal proportions. When nickel anodes only are being used, zinc must be added to the bath from time to time to strengthen the zinc salts and replace the zinc plated out. When it is possible to use zinc anodes alone the nickel is replenished in the nickel salts of the above character by adding nickel to the bath from time to time. Insoluble anodes may be used in some instances in the baths, but even when such electrodes are satisfactory'the bath requires the constant addition of both zinc and nickel'to replenish the metal concentration which has been depleted by being plated out of their salts.

During the RR. cycles in the bath, the periods of cathodic current last at least about 3 seconds apiece, but never longer than for about 30 seconds duration. The companion anodic pulses during the RR. cycle are intentionally shorter than the cathodic pulses, particularly when the anodic current intensity used exceeds the strength of the cathodic current used, but in any case the coulombs applied to the bath during each cathodic pulse exceed the coulombs during the subsequent anodic pulse to insure that the thickness plated each'time exceeds the amount deplated. Thus, a succession of net deposits is consistently accumulating from being cycli- "cally plated out of the solution and gradually building up as microscopically thin parallel plated increments on the article. The coulombs of each anodic pulse are 4 between about 20% and about of the coulombs of the adjacent cathodic pulse for best results.

In instances where the cathodiccurrent intensity and the anodic current intensity of the RR. period are each equal to approximately 50 amps. per sq. ft., the preferred plating cycles have had the proportions in the range between 12 seconds cathodic-9 seconds anodic (anodic coulombs 75% of cathodic) to about 10 seconds cathodic2 seconds anodic (anodic coulombs 20%) and the l2-second cathodic8-second anodic (anodic coulcmbs '66%%) and 13 seconds cathodic9 seconds anodic (anodic coulombs 69%) cycles Within that range have proved very satisfactory for the RR. process. In this range the alloys deposited have been between 56% and 96% nickel and the balance zinc.

In the following examples all baths, current densities, and temperatures are mainly the same. During each period captioned D.C. plating (Direct Current Alloy Plating) and likewise during each period captioned P.R. plating (Periodic Reverse Current Alloy Plating), the plating bath contained with minor variations:

The bath was bulfered by 2 /2% glacial acetic acid by volume. The respective amounts of NiCl -6H O and the ZnCl vwere the equivalents of 65 grams per liter of nickel metal and 50 grams per liter of zinc metal. The bath also contained 1% of Udylite Non Pitter No. 22. The bath had a pH of 2.2 and a temperature of F.

Nickel anodes were used.

' As a general guide regarding the nickel-zinc alloy deposits, it is to be kept in mind in the examples below that the DC. plating current and that the RR. current in both directions is very satisfactory in the range of 50-60 amps. per sq. ft. although the RR. currents in one direction, for example to make the article cathodic, is not necessarily equal to, but may for instance, be onehalf the strength of the RR. current in the anodic direction. Except as otherwise pointed out below the zincrich alloy applied during the DC. plating period is at least 0.1 mil thick, but preferably 0.4 mil thick and .good results have been obtained with between 0.6 mil and 1.2 mils thickness. In fact, there seems to be no readily ascertainable upper limit to the thickness of deposits which uniformly show good .adherence. Except as otherwise pointed out below the RR. period for applying the nickel-rich alloy lasts at least approximately 3 'minutes (resulting deposit 0.067 mil thick) and although the thickness can be slightly more or very substantially more, we preferably use a 10-minute RR. period which insures suflicient thickness (0.200 mil) that the deposit will adhere properly and without being porous as it would be if extremely thin.

The difilculty with porosity in the nickel-rich deposit is that it will admit chrome plating electrolytes and nickel plating electrolytes to contact the zinc-rich alloy under coat and in case such an article is immersed to be nickel plated or chrome plated, the electrolyte will immediately commence to dissolve out the Zn-rich alloy under coat and thus undermine the nickel-rich over coat in spots at the same time at which a nickel plate or chrome plate is presumably being applied over the entire surface.

The following examples illustrate various ways in which our process can be carried out.

Example I 'of nickel or copper plate.

Step one, a 29-minute period of DC. plating: The purpose of this step was to produce a zinc-rich alloy deposit. A current density of 50 amps. per sq. ft. was used and a buffered chloride bath of the foregoing constituents was employed. The zinc-rich alloy consisted of approximately 85% zinc and 15% nickel and the deposit was generally homogeneous and was 1.2 mils (0.0012") thick.

Step two, a -minute period of RR. plating: The purpose of this step was to produce an over coat of nickelrich alloy deposit and to do so the article remained in the same bath as in step one. The periodic reverse cycle was so arranged that this article was 8 seconds anodic-12 seconds cathodic, 8 seconds anodicl2 seconds cathodic, etc., recurrently for the duration of the 10-minute period. The current density was 50 amps. per sq. ft. and the thickness of the alloy deposit was 0.2 mil (0.0002). The nickel-rich deposit was generally homogeneous consisting of 80% nickel and 20% zinc.

Step three, buffing period: In this period buflfing compounds were abraded against the article to make it bright and also to remove any oxide coating remaining on the article and thereby reactivate its surface following plating.

Step four, removal of the buffing compound: The bufiing compound was removed by employing a cathodic cleaning step in which the article was made cathode in an alkaline cleaner, for example, an aqueous solution of 4-6 oz. per gallon caustic soda or sodium carbonate.

Step five, water rinse: in this step the article was rinsed in water to remove the bufifing compound cleaner.

Step six, acid dip: The article was dipped in a 10% solution of hydrochloric acid to reactivate the surface.

Step seven, Water rinse: This step of water rinsing the article insured the removal of any traces of the hydrochloric acid.

Step eight, chrome plating period: A chromium sulfate plating bath was used as the electrolyte which contained:

Chromic acid 45 oz. per gal. (01'0 Sulfate 0.45 oz. per gal. (S04).

The chrome plating bath was maintained at a temperature of 118 F. and the cathode current density was 100 amps. per sq. ft. The composition of the resulting product was chromium upon nickel-rich deposit upon zinc-rich deposit upon the base ferrous article. The period for depositing the chromium lasted time-wise purely according to the thickness desired which in this particular case was 0.015 mil (0.000015") thick. The foregoing method upon elimination of the chrome plate produced in step eight, with or without the elimination also of the buffing produced in step three, nevertheless produces more than a mere transitory stage intermediate article. Such article, sheathed in the outer coat of nickel-rich alloy of step two, satisfies a specific object of this invention of making it possible to electroplate nickel-zinc alloys on a commercial basis, and such nickel-rich electrodeposition of this character has not to our knowledge been previously done successfully. The bare nickel-rich deposit greatly improves corrosion resistance and, in the approximate thickness specified, is not porous as evidenced by the fact that it effectively insulates the zinc-rich under coat from its natural tendency of dissolution in chrome plating baths or other chemical agents which might attack it. The nickel-rich deposit manifestly offers corrosion proofing plus brightness utility when applied as the outer coat to articles lacking the immediately adjacent intermediate surface of zinc-rich alloy, but when the article is ferrous and also has a satisfactory thickness of the intermediate zinc-rich deposit under the nickel-rich outer coat, then a particularly useful and corrosion resistant, smooth and bright article results.

Example 11 The steps in this example are the same as in Example 1 above, except that following the completion of step two thereof and prior to beginning butfing step three, a light nickel electrodeposit was applied to the nickel-rich deposit on the bumper. In the known method, the nickel is applied in a manner to have dull semi-bright, or bright finishes optionally. The thickness of nickel deposit is preferably between 0.050 mil and 0.750 mil and in the present instance was approximately 0.1 mil (0.0001) thick. The nickel plate bath had a pH of 3.5 and contained, per gallon thereof:

45 oz. NiSO -6H O 5 oz. NiCl -6H O 5.5 oz. Boric Acid and the traces of this bath were then removed from the article by a conventional water rinse following the nickel plating. The resulting article was essentially the same as in Example I, the bufiing step removing some, but not all of the intermediate nickel coat from the article which therefore consisted of chromium upon nickel upon nickelrich alloy upon zinc-rich alloy upon the base ferrous article. The nickel protects the nickel-rich alloy from the efiects of buffing by forming a tough intermediate sheath which very satisfactorily holds up against the abrading effect of the buffing step and does so much more effectively than the 0.2 mil deposit of nickel-rich alloy does alone in its bare form as it results from step two in the Example I foregoing. The method of Example II, upon elimination of the chrome plate with or without the elimination of the nickel coat and also the buffing procedure, nevertheless produces more than a mere transitory stage intermediate article. Such article is of evident utility if the nickel coat is the outermost coat and, with the nickel outermost, this article has utility even if no zinc rich layer is present below the nickel-rich alloy deposit.

Example 111 Step one, a 29-minute period of DC. plating: The purpose of this step was to produce a zinc-rich alloy deposit on the bumper. A current density of 60 amps. per sq. ft. was used and a buifered chloride bath of the foregoing constituents was employed. The zinc-rich alloy consisted of approximately 85% zinc and 15% nickel and the deposit was generally homogeneous and 1.5 mils (0.0015) thick.

Step two, a 10-minute period of P.R. plating: The purpose of this step was to produce an over coat of nickelrich alloy deposit on the article which remained in the same bath as in step one. The periodic reverse cycle was so arranged that this article was 8 seconds anodic, 12 seconds cathodic, 8 seconds anodic, 12 seconds cathodic, etc., recurrently for the duration of the 10-minute period. The current density was 50 amps. per sq. ft. and the thickness of the plate was 0.2 mil (0.0002"). The nickel-rich deposit was generally homogeneous consisting of nickel and 20% zinc.

Step three, a 1-minute period of DC. plating: The purpose of this precautionary step was to depolarize the surface of the article through the application of a thin zinc-rich alloy deposit. The interposition of this deposit prevents an oxide film from forming on the article which would otherwise tend to form in some instances when it is initially exposed to the air following its polarization in the bath.

Step four, acid dip: The article was dipped in a 10% solution of hydrochloric acid to remove the thin zincrich deposit which at this point had fulfilled its entire purpose inasmuch as thereafter no oxide film has a tendency toward forming when the article is reexposed to air.

Step five, water rinse: In this step the article was rinsed in water to remove the hydrochloric acid.

Step six, bufiing period: In this period buffing compounds were abraded against the article to make it bright and smooth.

Steps seven-eleven complete the article and were identical with and in the same order as the last five steps (steps four-eight) of Example I, namely, removal of buffing compound, water rinse, acid dip, water rinse, and chrome plating period. The resulting article was essentially the same as in Example I, being composed of chromium upon nickel-rich deposit upon zinc-rich deposit upon the base ferrousarticle. The bond between the chromium and the nickel-rich deposit was highly eifective and tenacious owing to the eifect of the temporary sheath of zinc-rich alloy which during the process of its application and subsequent complete removal served the very effective purpose of insulating the article from the formation of any oxide film over the nickel-rich deposit, prior to the subsequent application of the chrome electrodeposit directly to the nickel-rich alloy deposit. It is equally advantageous to employ this zinc-rich temporary sheath when the nickel-rich under coat therebeneath is to be plated directly with nickel. That is to say, the resulting bond between the nickel-rich deposit and the over coat of nickel will be highly tenacious much in the same manner as the bond between the nickel-rich deposit and the over coat of chromium just noted.

Example IV Step one, a 4-minute period of DC. plating: A current density of 50 amps. per sq. ft. was used and a buffered chloride bath of the foregoing constituents was employed. The resulting zinc-rich alloy deposit on the bumper was of a thickness of 0.000166" (0.166 mil) and consisted of approximately 82% zinc and 18% nickel.

Steptwo, a 48-second period of P.R. plating: The purpose of this step was to produce a thin over coat of the nickel-rich deposit on the article which remained in the same bath as in step one. The complete 48-second periodic reverse cycle was based on the previously considered intervals of 8 seconds anodic, 12 seconds cathodic, etc., starting at random either somewhere in direct or in reverse, for instance 4 seconds anodic, 12 seconds cathodic, 8 seconds anodic, 12 seconds cathodic, 8 seconds anodic to a final 4 seconds cathodic interval totalling in all cases the required 48 seconds running time. The .current density was 50 amps, per sq. ft. and the very thin nickel-rich alloy deposit which resulted (0.000016) consisted of about 80% nickel and 20% Zinc. It is noteworthy, both at the-point of these first two steps in the present example as well as in the duplicative pairs of steps thereof to follow, that the alternate layers made of the 80% Ni-20% Zn binary alloy in the deposit can :be considered as microscopically thin (:016 mil) compared to each remaining layer composed of the 0.166 mil thick, 18% Ni-82% Zn binary alloy which latter thus amounts to being slightly more than ten times thicker, layer or layer. This disproportionate relationship of the discrete, microscopically-thin, P.R. increments between the successive layers of the parallel-plated, zincrich alloy accounts fora non-homogeneous deposit of the alloy averaging in proportions at value roughly of 80% zincand 20% nickel.

Step three, a 4-minute period of DC. plating: The article remained in the same bath as in steps one and two and this third step is identical with step one, thereforeresulting in an over coat of the zinc-rich deposit (85% zinc and 15% nickel) upon the very thin nickelrich alloy deposited in step two.

Step-four, a 48-second period of P.R. plating: This step Was identical to step-twoand the article remained in the same bath.

Step five, a 4-minute period of DC. plating: This step wasidentical to step one and the article remained in the same bath.

Step six, a 48-second period of P.R. plating: This step was identical with step two using the same bath.

Step seven, a 4-minute period of DC. plating: This step was the same as step one and performed in the same bathr Step eight, a '48-second period of P.R. plating: This 8 step was the same as step two and performed in the same bath.

Step nine, a 4-minute period of DC. plating: This step was the same as step one and performed in the same bath.

Step ten, a 48-second period of P.R. plating: This step was the same as step two and performed in the same bath.

Step eleven, a 4-minute period of DC. plating: This step was the same as step one and performed in the same bath to produce a zinc-rich alloy deposit (0.166 mil) of 82% zinc and 18% nickel. Thus, the first eleven steps in this example consumed approximately 28 minutes which roughly corresponds with the 29-minute period of the single step one of Examples I, H, and III foregoing. if we consider this example from standpoint thus far of its 28-minute multi-layer deposit vs. the more conventional 29-min. straight D.C. periods of deposit just noted, it compares favorably timewise (28 min. vs. 29 min.), current density-wise (50 a.s.'f. vs. 60 a.s.f.), also thickness-wise (1.080 mils Vs. 1.200 mils), and proportionwise (/20% avg. Zn to Ni vs. 85/15% Zn to Ni respectively). However, the alternate nickel-rich barriers included down in the multi-layer type deposit produce decidedly superior results not the least of which is the prevention of grain growth and cracks; these results would of course be impossible of accomplishment as a practical matter without using the same bath for the 28- minute period hereof or without using the interspersal of short P.R. cycles or both. Alternately for instance, it would be grossly uneconomical to alternate the article between adjacent D.C. plating tanks of straight nickelrich binary plating electrolyte and straight zinc-rich binary plating electrolyte to produce the grain-growth preventing barriers interspersed at the right intervals aforesaid, and to do so would be impractical timewise or either from the strict economy stand-point if not both.

Step twelve, a 10-minute period of P.R. plating: The purpose of this step was to produce a moderately thick overcoat of nickel-rich alloy deposit and the article remained in the same bath as in steps one-eleven. The periodic reverse cycle was arranged whereby the article was 8 seconds anodic, 12 seconds cathodic, 8 seconds anodic, 12 seconds cathodic, etc., in series recurrently for the duration of the 10-minute period. The current density was 50 amps. per sq. ft. and the thickness of the alloy deposit was 0.2 mil (0.0002). The nickel-rich deposit was generally homogeneous consisting of 80% nickel and 20% zinc.

lt bears noting, both at this point of the present example as Well as in the previous examples, that plating vs. deplating time within the stated P.R. cycle yields a net of 4 seconds out of every twenty seconds, amounting to a plating time excess of 12 seconds per minute of operation, i.e., 2 minutes net plating accumulated out of the full 10-minute period of P.R. operation. The total deposit which accumulated was 0.200 mil in thickness and in step eleven preceding, the DC. deposit thereof resulted at a rate of 0.166 mil mil/mm.

For a comparative two minute period, the calculated the actual P.R. circumstances of two minutes net plating time. This apparent discrepancy is probably best explained on basis of polarization effects as chief factor. By the same calculations, the 48-second P.R. period of step two would appear to yield 0.0055 mil by net plating time Whereas the actual thickness was 0.016 mil by microscope.

Step thirteen, a l-minute period of DC. plating: The purpose of this step was to depolarize the article to prevent the formation of an oxide film which might already exist or otherwise might tend to form thereon when the article is initially exposed to air following its removal from the bath with possible residual traces of polarization.

Step fourteen, acid dip: The article was dipped in a 10% solution of hydrochloric acid to remove the outermost zinc-rich alloy deposit, the situation being that the outermost deposit has completely fulfilled its purpose of eliminating the tendency for oxygen film to form on the resulting nickel-rich alloy.

Step fifteen, water rinse: This step of water rinsing the article was to remove the hydrochloric acid.

Step sixteen, nickel plating: A light nickel electrodeposit was applied to the article to a thickness of 0.1 mil (0.0001") thick. The nickel plating bath had a pH of 3.5 and contained per gallon thereof:

45 oz. NiSO -6I-I O OZ. 5.5 oz. Boric Acid The light nickel electrodeposit had the purpose of protecting the nickel-rich alloy from the effects of boiling, by providing thereover a tough intermediate sheath able to withstand bufling during the step immediately below.

Step seventeen, buffing period: In this period butfing compounds were abraded against the article to make it bright and smooth.

Steps eighteentwenty-two are the same five steps (steps four-eight) of Example 1; namely, removal of the buffing compounds, water rinse, acid dip, water rinse, and chrome plate period. It will be noted that Example IV incorporates the temporary zinc-rich alloy sheath and acid dip (steps thirteen, fourteen) that are involved in Example III in steps four and five. It will be further noted that Example IV incorporates the protective nickel sheath and the bufling step (steps sixteen, seventeen) that are in Example II wherein the nickel sheathes the nickelrich deposit against the wearing effect of the buffing step. It will be further noted that Example IV incorporates the last five steps of Example I leading up to the chrome plating step which is optional in all examples. Example IV, however, in distinction to the previous examples provides thin boundary layers of nickel-rich alloy deposit (steps two, four, etc.) between successive thick zinc-rich deposits (steps one, three, etc.) to prevent grain growth and spreading of cracks between these Zinc-rich deposits. Actually these boundary layers are a homogeneous nickel-rich deposit composed of an imperceptible or barely perceptible plurality of microscopically thin plated increments. The composition of the resulting product of Example IV was chromium upon nickel upon a single layer protective coating consisting of 0.200 mil thick nickel-rich alloy (moderately thick) upon an eleven-layer, 1.080 mils thick coating portion consisting of respective increments of zinc-rich alloy (moderately thick) upon nickel-rich alloy (very thin boundary) upon zinc-rich alloy (moderately thick) upon nickel-rich alloy (very thin boundary) upon Zinc-rich alloy (moderately thick) upon nickel-rich alloy (very thin boundary) upon zinc-rich alloy (moderately thick) upon nickel-rich alloy (very thin boundary) upon zinc-rich alloy (moderately thick) upon nickel-rich alloy (very thin boundary) upon the initial increment of zinc-rich alloy (moderately thick), all upon the base ferrous article.

The method of Example IV, upon elimination of the chrome plate step with or without the elimination also of the bufiing and/or the nickel plate, nevertheless produces more than mere transitory stage intermediate articles. The included barriers of nickel-rich alloy according to steps two, four, six etc., between the thicker zincrich barriers of steps one, three, five, etc., prevent crystal growth and the spreading of cracks between the thicker layers of the corrosion resistant zinc-rich deposits and thus possess broader application and have far more general utility than merely in the article ultimately completed in Example IV. More specifically, in articles of this general utility, the nickel-rich alloy boundary layer should be applied to an average thickness of at least approximately 0.000006" (0.006 mil) to produce a barrier with the desired characteristics, it being unimportant except from a time and cost standpoint as to what the upper limits of the thickness are. Usually however, the thickness of nickel-rich boundary layer is a value between about 0.000003" and about 0.0002", a preferred thickness for the boundary layer being approximately 0.00002 (0.02 mil).

The successive layer of the D.C.-deposited zinc-rich alloy on each boundary layer is no thicker than approximately 0.0006 (0.6 mil). Although there is no recognizable lower limit to the thickness of the zinc-rich layer, the desire obviously is in the other direction, so as to materially build the deposit in thickness under every one of these D.C. applications. Usually the zinc-rich alloy has average thickness of a value between about 0.0001 and about 0.0004, a preferred thickness of zinc-rich layer being 0.0002 (0.2 mil); otherwise the boundary barriers do not interpose frequently enough to interrupt ,and be successful against the grain growth and spreading of cracks in those zinc-rich layers. In view of the fact that the preferable nickel content in each of the latter layers has a value in the stated range of about 13% to about 24% nickel and balance Zinc, the entire lamellar deposit averages the overall preferred value of about 1 6% to about 31% nickel by weight and the balance zinc.

Example V A zinc die casting for use as an automobile headlight ring was subjected to the following steps:

Step one, polish and buff with proper compound.

Step two, alkaline cleaning step employing electrocleaning operation with appropriate alkaline cleaner to remove the polishing and bulfing compounds from the die casting.

Step three, water rinse.

Step four, acid dip in 2% sulfuric or 2% mu-riatic acid or combination of both.

Step five, water rinse.

Step six, 29-minute period of continuous D.C. plating: This die casting was electroplated in a buflered chloride bath of the foregoing constituents to produce a zinc-rich alloy deposit at a current density of 50 amps. per sq. ft. The thickness was 0.0012" (1.2 mils) of simultaneously deposited zinc and nickel in the proportions Zinc and 20% nickel.

Step seven, butting step: This step is optional, and requires that the article be temporarily removed from the foregoing buffered chloride bath.

Step eight, alkaline cleaning step employing electrocleaning operation with appropriate alkaline cleaner to remove the bufling compound of the preceding optional step from the die casting: This step is, of course, optional depending solely on whether the optional bufiing step was used. 1

Step nine, acid dip: The article was subjected to an acid dip in a proper bath to neutralize the alkaline treated surface thereof.

Step ten, 2-minute step of continuous D.C. plating: Th s optional step reconditions the buffed surface of the article to essentially the status of what that immediate surface would have possessed had steps sevennine been omitted following the D.C. plating step six. This step ten is highly desirable where the buffing and dipping steps sevennine are employed, and, of course, involves the reintroduction of the article into the same or equivalent bath as the buffered chloride bath of step six from which it was temporarily removed.

Step eleven, IO-minute P.R. stepzThe article was subjected to 50 amps. per sq. ft. current density employing the RR. cycle in the same bath as in the step six preceding, the intervening steps seven-ten being omitted or not as optional. The co-deposited nickel-rich nickel-zinc was in the proportion 80% nickel, 20% zinc to a thickness of 0.2 mil (010002").

Step twelve, l-minute step of continuous D.C. plating: Thisstep prevents the formation of oxides when the article is first exposed to air upon removal from the bath followingv step eleven.

Step thirteen, acid dip: The article was subjected to an acid dip. in a bath having a concentration of 10% HCl to remove the zinc-rich deposit formed in step eight thereby making the external nickel-rich coating acid to receive adeposit and actively bond thereto in the following step.

Step fourteen, nickel plate: The article was nickel plated either semi-bright or bright to a thickness of 0.0003 (0.3 mil) or 0.000 (0.4 mil). The bath employed had a pH of 3.5 and contained per gallon thereof:

OZ. Nickel chloride Nickel sulfate 45 Boric acid 5.5

Step fifteen, bufling step: The article was bufied in the case of the semi-bright nickel plate of the preceding step although bufling is unnecessary with the bright nickel plate option of the preceding step fourteen.

Step sixteen, chrome plate step: The article was chrome plated to a thickness of 0.010 mil (1 0 millionths of an inch) and the chrome plating electrolyte consisted of:

Oz./gal. Chromic acid 45 S0 0.45

The resulting article was chrome plate upon nickel plate upon nickel-rich alloy upon Zinc-rich alloy upon the base article which was a zinc die casting. The purpose of the P.R. step six was to enable the nickel plate according to step fourteen to adherently bond to the zinc die casting without the necessity for a cyanide copper over coat in order to properly take the bright nickel plate.

The multiple step procedures and the respective prodnets of the preceding examples are not exhaustive. In many cases the plating current employed will be satisfactory under various ranges of impressed voltage depending upon the spaced relationship of the anodes and the articles treated, and relatively low plating and deplating voltages were actually employed which were in the range between 6 and '24 volts and preferably the plating and deplating voltages were each about 10 volts D.C. In each case the anodes were nickel, but the plating and deplating can be accomplished satisfactorily according to this invention with electrodes of other compositions.

The electrodeposits produced by the invention are particularly applicableto automobile bumpers, hardware, and the like which are subject to abrasion and exposure to moisture and deleterious gases. The electrolyte may be agitated and distributed in accordance with wellknown practices in plating. It has been found desirable to maintain .a clean electrolyte, for example, by filtering or the like, inasmuch as' the electrodeposits have such a smooth and highly polished surface that any solid impurities from the electrolyte deposited on the plated work are much more apparent than in ordinary electroplating.

When used in the claims, the term cathode current density shall be construed to refer to the current density at the article being electroplated. Thus, this term will also cover the current density at the anode during the.

anodic phases of the periodic reversal cycle. Percentages of ingredients recited in the claims are percentages by weight.

Variations withinthe spirit and scope of the invention describedare equally comprehended by the foregoing description,

We claim:

1. A metal article comprising a base composed chiefly of a metal selected from the group consisting of iron and zinc, said base having an adherent protective coating of alloy consisting of a layer of electrodcposited nickel and Zinc alloyed with one another with average proportions of between about 13% and about 24% nickel and the balance Zinc, and an adjacent alloy deposit directly thereupon having average proportions of between about 56% and about 96% nickel and the balance zinc.

2. A metal article comprising a base member made chiefly of a metal selected from the group consisting of iron and zinc, said base member having an adherent protective coating of binary alloy consisting of a layer of electro-deposited nickel and zinc of average proportions of about 20% nickel and the balance zinc, and an outermost deposit upon and adhered directly to the just named layer having average proportions of nickel and 20% zinc approximately.

3. A ferrous metal article having an adherent protective coating consisting of a layer of electro-deposited nickel and zinc alloy of average proportions of 80% zinc and 20% nickel approximately, an adjacent alloy deposit thereover having reversed proportions to the foregoing so as to average 80% nickel and 20% zinc approximately, and an outermost elemental metal layer of nickel plate.

4. The protective coated article of claim 3 wherein said elemental metal layer of nickel carries an adherent exposed elemental metal coat of chromium plate of predetermined thickness.

5. As a new article of manufacture, a ferrous base member having an adherent protective coating consisting of an alloy layer of nickel and zinc simultaneously deposited from a single electrolyte and containing an average of about 80% zinc, another alloy layer consisting of nickel and zinc simultaneously deposited on the zincrich layer from the same electrolyte and containing an average of about 80% nickel, and an outermost exposed layer of elemental metal consisting of electro-deposited chromium on the nickel-rich alloy layer. I

6. A metal article selected from the group consisting of iron and zinc having an adherent protective coating predominantly 80% zinc and 20% nickel on the average, said protective coating having a plurality of microscopically-thin, spaced barrier coats of nickel-rich alloy down in the deposit between successive layers of parallel-plated, zinc-rich alloy to prevent crystal growth and the spreading of cracks between said successive Zinc-rich layers.

7. An article having the adherent protective coating of claim 6 wherein each barrier coat comprises approximately 80% nickel and 20% zinc and has a thickness of a value between about 0.003 mil and about 0.200 mil, said successive layers of the Zinc-rich alloy being of substantially reversed proportions to the above and at least approximately ten times thicker.

8. Iron or steel articles composed or having a surface of iron or steel furnished with a non-homogeneous coat ing or deposit of nickel-zinc alloy containing an average of about 80% Zinc by weight and the balance nickel, said non-homogeneous alloy consisting of pluralities of incremental metal layers alternating with one another, one plurality of said alloy metal layers being of relatively major thickness and separated by included grain-growth preventing barriers formed by microscopically-thin parallel-plated alloy increments of another plurality of said layers each barrier containing nickel of an amount between about 56% and about 96% by weight with the balance zinc.

9. The article defined in claim 1 wherein said adjacent alloy deposit is at least approximately 0.067 mil thick.

10. The article defined in claim 1 wherein said adjacent alloy deposit is at least approximately 0.2 mil thick.

11. A ferrous metal article having thereon at least 13 two directly super-posed thin electrodeposited coating layers, the lowermost of said layers consisting essentially of 'about 80% to 85% zinc and about 20% to 15% nickel, and the uppermost of said layers consisting essentially of about 80% nickel and about 20% Zinc. 5

References Cited in the file of this patent UNITED STATES PATENTS 1,564,581 King Dec. s, 1925 10 14 Fink Feb. 10, 1931 Huston Dec. 24, 1940 Murray July 27, 1943 Schantz Apr. 22, 1947 Tietz May 31, 1949 Berman Oct. 27, 1953 Hammond Apr. 27, 1954 Wesley July 22, 1958 Kenmore Dec. 29, 1959 

1. A METAL ARTICLE COMPRISING A BASE COMPOSED CHIEFLY OF A METAL SELECTED FROM THE GROUP CONSISTING OF IRON AND ZINC, SAID BASE HAVING AN ADHERENT PROTECTIVE COATING OF ALLOY CONSISTING OF A LAYER OF ELECTRODEPOSITED NICKEL AND ZINC ALLOYED WITH ONE ANOTHER WITH AVERAGE PROPORTIONS OF BETWEEN ABOUT 13% AND ABOUT 24% NICKEL AND THE BALANCE ZINC, AND AN ADJACENT ALLOY DEPOSIT DIRECTLY THEREUPON HAVING AVERAGE PROPORTIONS OF BETWEEN ABOUT 56% AND ABOUT 96% NICKEL AND THE BALANCE ZINC. 